Tomatoes to the rescue!
A parasitic plant that devastates crops in some parts of the world may now have met its match thanks to the household tomato. Cuscuta reflexa invades the stems of its hosts and steals water and nutrients. This damages the yield and also leaves the victim susceptible to infection by other plant pathogens. But, by chance, tomato plants have naturally evolved a mechanism to detect the parasite and, when it tries to invade, they kill it. Markus Albert, who made the discovery, has now uncovered how tomatoes do this, and thinks it might be possible to endow other crop plants with the same defence system, as he explained to Chris Smith...
Markus - This is a plant but it's also a parasite which attacks other plants. This plant has no roots, it has no leaves, so it requires nutrients from other plants. It grows inside the stem of another plant and then it starts to withdraw water, nutrients, and also carbohydrates.
Chris - What does the parasite look like then this parasitic plant, before it gets inside the host plant?
Markus - So the parasite is a winding plant that winds around stems of host plants. Before it starts to penetrate it has a special organ called 'haustorium' - a kind of a root that grows inside the host plant, and then it connects directly to the tissue where water flows. So it makes a continuous tube from the host to the parasite and the parasite is getting everything out.
Chris - What sort of impact worldwide does this class of parasite have on plants that we judge to be important - crop plants, for example?
Markus - In Southern Asia it's a very dangerous plant in the soya bean cultivation. Even for some tomatoes it can be very dangerous or it decreases the crop yield tremendously. And also in coffee cultivation in can be a really huge problem. The biggest problem - you cannot fend off this parasite by using herbicides because, otherwise, you would also kill the host plant or the crop plants.
Chris - How does it spread then - does it make flowers at some point and set seed?
Markus - It makes flowers and this flowering time is somehow synchronized with the host plant. These parasitic seeds, they can stay in the soil for years.
Chris - And are all plants, as far as we know, susceptible to this parasite or are there some that have evolved to fend it off?
Markus - Nearly all plants are susceptible to this cuscuta species, but we noticed that the cultivated tomato is resistant against this parasitic attack and there we observed an active defence. The site where this parasitic penetration organ aims to penetrate the tomato stem, the surrounding tissue becomes kind of brownish, and what we then observed when we made a cut through the stem, this haustorium could not grow inside. And very important, about two weeks later, the parasite dies.
Chris - So just by chance, it would appear, the tomatoes that we grow because we love to eat them appear to have some kind of mechanism that can fend off this parasite - it stops it plugging into the vascular system of the plant. How did you then pursue that to work out how it's doing that?
Markus - We also noticed that a wild tomato species was susceptible. Other people crossed the two tomato species and provided us a set of different tomato strains with a diverse genetic background, and these were all tested and this helped us to further characterise and identify important components.
Chris - I get it. So by comparing a strain of plants that are susceptible to the parasite with a strain of plants that are not susceptible to the parasite you can then ask well, what are the differences between the two? Because the differences must hold the key to that resistance and then you can begin to pick through and see what exactly is missing from the plant which is susceptible and that tells you. So what is it - how is the tomato plant fending off the parasite?
Markus - We mapped a certain region of this tomato genome and we ended up with just one important component and this is a kind of receptor mechanism. This receptor could detect a molecular signature from this plant parasite and then it works like a switch and starts a kind of signalling cascade that leads to this defence.
Chris - Would this tripwire system be amenable to being transferred to other plants that are currently susceptible to the parasite? So could you put the same machinery into soya beans, for example, so if the parasite tried to get in there, then the soya bean would also be able to fend it off?
Markus - So, I think this is something important we aim to do. What we initially tried - we transferred this machinery to two other plant species and there we could see the resistance increasing against this parasite. And now it will be an important aim to transfer this also to important crop plants, for example, soya bean or some others.